Added simple filter queries for collections in the server API. Such
queries should cover simpler cases, while more complex filters should
use the JSON syntax.

- Added simple filters for collection endpoints in API (e.g.
`/tasks?project_id=42&user=myusername`)
- Removed duplicating collection endpoints in API in favor of their full
versions:
  - `/projects/{id}/tasks` -> `/tasks?project_id={id}`
  - `/tasks/{id}/jobs` -> `/jobs?task_id={id}`
  - `/jobs/{id}/issues` -> `/issues?job_id={id}`
  - `/issue/{id}/comments` -> `/comments?issue_id={id}`
  - Corresponding owning objects now return a link to the collection:
    - `/projects/{id}`: `.tasks`
    - `/tasks/{id}`: `.jobs`
    - `/jobs/{id}`: `.issues`
    - `/issue/{id}`: `.comments`
- Fixed errors in the generated server API schema:
- Input and output types in all the basic model methods (`Create`,
`Update`, `Retrieve`, `Delete`)
- Removed invalid separation for the project list operation response
type
- File structure for projects in the dataset cache is updated to store
the new project representation. Old caches will be invalidated.
- Added tests